/*
There are N gas stations along a circular route, where the amount of gas at station i is gas[i].

You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from station i to its next station (i+1). You begin the journey with an empty tank at one of the gas stations.

Return the starting gas station's index if you can travel around the circuit once, otherwise return -1.

Note:
The solution is guaranteed to be unique.
*/

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int nums = gas.size()*2;
        vector<int> res(nums,0);
        for (int i = 0; i < nums; i++) res[i]=gas[i%gas.size()]-cost[i%gas.size()];
        int start = 0, cur = 0;
        for (int i = 0; i < nums; i++) {
            if (cur < 0) {start = i; cur = 0;} 
            cur += res[i];
            if (cur >= 0 && (i-start+1) >= gas.size()) return start;
        }
        return -1;
    }
};
